Evaluation of an Old Coal Mine in the area of GeothermieZentrum Bochum by Geophysical Prospection

Field Measurments in Search of Empty Spaces or Weathered Material in the Subsurface


Master's Thesis, 2016

60 Pages, Grade: 2.0


Excerpt

Table of Contents

Abstract

1. Introduction
1.1 Motivation
1.2 Research Area
1.2.1 Geology
1.2.2 Former Studies

2. Surveys and Theoretical Background
2.1 Ground Penetrating Radar Survey
2.2 Geoelectric Survey
2.3 Seismic Survey

3. Field measurements
3.1 Measurements at the western side of GZB
3.2 Measurements at the eastern side of GZB

4. Data Processing and Results
4.1 Analyses of radargrams with ReflexW
4.2 Inversion of pseudo-sections with BERT and DC2DInvRes
4.2.1 Geoelectric data analysis of the western side of GZB
4.2.2 Geoelectric data analysis of the eastern side of GZB
4.3 Seismogram processing with FMTOMO
4.3.1 Analysis of the pre-existing seismic data near GZB building
4.3.2 Analysis of the seismic data from the survey at the eastern side of GZB

5. Interpretation
5.1 West of GZB
5.2 East of GZB

6. Summary and Conclusions

Acknowledgements

Bibliography

Appendix I

Processing workflow for ground-penetrating radar raw data with ReflexW

Appendix II

Processing geoelectric data with BERT

APPENDIX III

Processing seismic data with ActiveSeismoPick3D

Abstract

Geophysical measurements are often used to determine the state of the underground and to detect possible structural weaknesses. In contrast to the direct underground investigations, like drilling, the on-surface prospection is less expensive and less destructive. This study is intended to investigate the applicability of geophysical measurements for the determination of the subsurface stability of an old coal mine in the area of GeothermieZentrum of Bochum.

Former studies and reports of the selected area were taken into consideration to approximate the geological and structural condition of the subsurface. These reports incorporate information about the preceding restoration projects that took place along the last 40 years and help us gain a sufficient image about what is to be expected into the subsurface. This information was used in combination with the geophysical data, obtained during the current study, to attain results about the state of the ground and to create general implications for the suitability of geophysical investigations of old near-surface coal mines.

The used geophysical methods are the ground penetrating radar (GPR), geoelectric prospection, and seismic refraction. The collected data were processed and evaluated with several specialized programs, such as, ReflexW, Geotom, Bert, Res2DInv, and FMtomo. The maximum estimated penetration depth through the final inversion process is 30-40 m, depending on the type of the survey.

This study, aims to establish an evaluation workflow using geophysical measurements to prevent building hazards due to ground collapse, settling and subsidence in areas with former mining activity. This workflow should be applicable to locations with a similar historical background to the study site to attain public safety, providing an alternative, urban friendly and non-destructive inspection method.

1. Introduction

1.1 Motivation

The Ruhr-area used to be the leading producer of coal among all European countries, during the 18th,19th and 20th centuries. In the late 1990s the coal production was no more as profitable as it used to be and along with other factors, like health hazards that coal mining was accused for, they lead to dramatical decline of the mining activity and most of the mines have been closed. After the end of the exportation of the coal, the empty space that stays behind into the subsurface has to be filled up with the mining waste, and sometimes with pressed cement, so that ground stability is attained. For reasons that have not yet been clarified, in some areas, where a mine restoration by cement injection took place, the ground, nevertheless, underwent severe subsidence and depression. The crater of Wattensheid (an area which belongs to Bochum) is the nearest incident to the studied area. On the January of 2000, a crater of 500 m2 surface and 15 m depth along with a smaller crater near it, opened up in the urban area of Wattenscheid with no previous warning (DW, 2000). The incident is more likely connected to the activity of the old Mine “Marie nne”, which was operating until 1905. Although the regular restoration project, that follows the closing of every mine, took place, the incident could not be avoided.

The reason that caused the subsidence and ground collapse at that specific time, is not yet determined but the reason why the incident happened at that specific area and what it was related to was specified as connected to the previous mining activity. It is consequently important to inspect such areas which were restored after previously mining activity in urban areas and non. Firstly, in the sense of safety reasons and secondary, because of financial reasons. Until now, the most accurate method that is used for the inspection of the subsurface and the estimation of the weaknesses of the subsurface, is the exploration drilling. Nevertheless, it is a very destructive and expensive method which rules out the probability of using this method for the underground exploration just for precaution. The idea of using geophysical methods, such as electromagnetic and seismic tomography for the evaluation of the subsurface could provide a partial or complete solution to that problem. Even if the exact condition of the subsurface structures detected through these methods cannot be estimated, at least it could provide considerable results for targeted exploration drillings which would minimize the costs and the level of damages comparing to spread exploration projects along the whole area of interest.

Therefore, target of this study lays the attempt of evaluating the subsurface of an old mine around GeothermieZentrum Bochum (GZB) through several methods such as ground penetrating radar, geoelectric tomography and seismic surveys in order to propose a more convenient, less expensive and less destructive exploration method. If this method give considerable results for the evaluation of the state of the subsurface, it could be a good proposal for a precautious exploration workflow of after-mine- activity restored locations of the Ruhr area and on other regions, where the mining activity was near surface.

1.2 Research Area

The studied area is positioned at the western part of Germany in the state of North Rhine-Westphalia. It lays between the northern edge of the, so called, Rheinische Schiefergebirge, which are some slate mountains around the area of the Rhine river, and the Münsterländer Cretaceous - basins. More specific, Bochum is located on the formation of the slate mountains (Fig. 1).

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Fig. 1 Geological overview map of the Ruhr area. http://geopark.metropoleruhr.de

The Ruhr area used to be an active mining area for hard coal and iron ore. Once the mining activity ends, the mines have to be refilled with the material excavated from the mine and/or concrete in order to retain the surface stability. At these locations of the Ruhr area, which were close to the Rhine river and in the south of it, such as the Mutten valley close to the city of Witten, the region around the city of Sprockhoevel and in the southern suburbs of Bochum and Essen, the coal seams were folding upwards, and they were dipping in a certain angle to the surface so that they were easily uncovered after the top layers were eroded. In those areas, the coal exportation was easy and fast (Pro- Bergbau). The old coal mine “Markgraf 2”, which will be investigated, was operating in the area of GeothermieZentrum Bochum and used to be such a near-surface hard coal exportation mine.

1.2.1 Geology

The Rhenish Slate Mountains are a geological massif covering part of the western Germany, eastern Belgium, Luxembourg and northeastern France. It consists of metamorphic rocks, mostly slates (hence its German name, Rheinisches Schiefergebirge), deformed and metamorphosed during the Hercynian orogeny (around 300 million years ago). Most of the massif is part of the Rhenohercynian zone of this orogeny. The majority of the rocks in the Rhenish Massif were originally sediments, mostly deposited during the Devonian and Carboniferous in a back-arc basin called the Rhenohercynian basin. In the eastern Rhenish Massif, some very limited outcrops in the Sauerland show rocks of Ordovician and lower Siliurian age (Walter, 1992).

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Fig. 2 Geological-structural profile of the coal seam “Girondelle” (yellow),from a cross-section parallel to the area of GeothermieZentrum Bochum (GZB). (modified after Stempelmann 2010)

The investigation area is located on the northern flank of the Stockumer main anticline (Map: Witten-Coal Formations, Fig. 2). The exact position of the cross-section of Figure 2 lays 250 m western of the investigated area, between the Ruhr University of Bochum and the GZB. Nevertheless, it represents the general dipping of the layers and the relative position of GZB to the geological condition of the area. The layers strike from SW to NE. They are regionally governed by a steep, interchanging depositional dipping, in consequence of a repeated succession of anticlines and folds. More to the north, the traces of a thrust fault are documented in the area of Fachhochschule Bochum. The layers, in the north limp of the Oberstiepeler anticline, are dipping with an angle of 70° (± 5°) to the northwest. Because of local crenulation, the dipping direction might vary in places. The maximum elevation of the field is 124 m above sea surface.

The ground consists of a low-thickness cover of loose material and loess loam until the depth of 2-3 m. The poor consistence of the hummocky near-surface ground results from the agricultural use of the field in the late years. On the lower limit of the ground cover, an abrupt increase of the propagation velocity from 430 m/s to 840 m/s was encountered. This abrupt increase is present because, right below the top cover lays the carboniferous coal bearing bedrock formation. It constitutes by an interplay of fine grained claystone, siltstone and sandstone with coarse grained conglomerates. Among those successions occur several coal seams and layers with various percentages of coal. On the upper part of this formation the material is weathered for several meters, when below approximately 30 m the rock is intact. According to the geological maps and mine documentation for the current location, there are inconsistent statements concerning the actual stratigraphic sequence. However, there is no doubt that the coal seam with name “Sonnenschein” or “Girondelle” used to outcrop on the ground surface with direction SW to NE between those layers. The record of the log of the coal seam (from now on “Girondelle”) consists of a succession of 0.67 m coal, 0.43 m bituminous shale, 1.2 m coal, 0.25 m impure coal and 0.25 m coal. Hence, the coal seam constitutes of 2.8 m total thickness. Stratigraphically, this formation belongs to the Bochum Series of the Westfal A sequence.

1.2.2 Former Studies

The mining activity in the studied area stopped in 1958. The field name is “Mansfeld” and the mine was called “Markgraf 2”. In 1978, the Ruhr University of Bochum decided to use the area northern of the Bochum- Fachhochschule to build sport facilities for the department of sport sciences. The study contacted by the Westfälische

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Fig. 3 Restoration project for the building of sport facilities for the Ruhr University of Bochum. Approximate fitting of the report from 1980’s sketch to the google map image. The orange area represents the extent of the restoration.

Bergewerkschaft determined that the ground was not stable enough, rejecting the possibility for any building activity in the area. In 1980, a restoration project for the attaining of the ground stability took place. 245 drillings for injection and grouting from borehole to borehole were dug on a raster of 5 m spacing along the old coal seam. The actual positions of the injection points are absent. We only know the area which was generally covered by the restoration using a handmade skitch to fit it to the google map image (Fig. 3). The total volume of the injected material into the boreholes and the underground cavities is 3.232 m3. Though the restoration project was finished, the sport facilities building never took place.

In the framework of PROMETHEUS project which was aiming for geothermal energy, a second study took place in 2005, with the target of building a drilling platform and a deep borehole down to 4.000 m depth. The platform should be constructed southern of the old coal mine, into the big field, on the northern limb of the eastern working area of this study. Although this area seems to be out of the excavated area of the mine, the construction technical requirements for such a project were not fulfilled, since the subsurface properties indicated high probability for subsidence.

One important outcome of the PROMETHEUS project study concerns the safety pillar of the old coal mine. A pillar is located at a certain position into the extent of a mine, where there is no excavation activity taking place, to maintain structural stability along the mine. It is approximately located below the road Auf den Kalwes (Fig. 3), according to the report of 1980. This study concluded, that the integrity of the pillar could not be verified and that it was not possible to figure out whether it was affected by the old mine due to the poor documentation of the previous reports.

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Fig. 4 Restoration project for the building of GeothermieZentrum Bochum and GeoTechnikum hall. Approximate fitting of the report from 2010 to the google map image combined with the restoration Project of 1980. The orange area represents the extent of the restoration of 1980 and the blue area the extent of the restoration of 2010.

During the following years, a long-term study (2004-2009) concerning the stability of the subsurface took place, not only for the area below GZB, but also below the buildings of the Fachhochschule Bochum, regarding the possibility of preexisting pits also at this area (Schneider, 2009). In 2010, the Fachhochschule Bochum planned to build the facilities for the Institution of GeothermieZentrum Bochum and GeoTechnikum. The new buildings were planned to be located south of the last buildings of the Fachhochschule Bochum and along the area where the old mine used to operate. For this study, they created a raster of 10 m spacing as a coordinate system with letters (A-F) on the short axis and numbers (0-160 m) on the long axis. 24 exploration boreholes were drilled into this raster until maximum depth 39 m below surface. Ten of these boreholes were drilled with 45° inclination towards the SE. The exploration process revealed that the mining activity at this location reached the depth of 50 m below surface on the northern part and until the depth of 19 m on the southern part. According to the HOLLMANN & NÜRENBERG 1972 diagram, there is high possibility for collapse, settling and/or subsidence of the ground, when the dipping angle of the coal seam is 70° and the vertical distance of the top of the coal seam until the roof of the bedrock below, is less than 20 m, which was the case of the coal seam in the studied area. Thereafter, a second restoration project for the building of the new institution buildings in this area followed (Fig. 4). For the restoration project, the subsurface should be handled with cement suspension grouting, until 30 m depth, so that the closure and the stability of the loose masses could be ensured. The total number of injection boreholes, which were drilled for the second restoration project, were 165 and the total volume of injected material 239,1 m3. This number is not to be compared with the first restoration project because the covered area by the second restoration project differs largely from the first restoration project.

During the last restoration project, an extended study of 65 exploration boreholes took place, giving a sufficient image about the composition of the subsurface. As mentioned above, the positions of the exploration drillings were placed over a rectangle raster of 10 m spacing. It was 160 m long and 50 m bright. The long side is parallel to the coal seam direction (WSW-ENE) (Fig. 5). The information about the geology of the

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Fig. 5 Area covered by the exploration drillings of the restoration project 2010. The yellow area represents the explored area. The raster is not visible on this figure. The points represent the drilling positions and the red line represents the modeled cross-section through Rockworks 2006.

investigated area, gathered by the exploration drillings of the restoration project of 2010, is very precise and consistent since the exact positions of each drilling were pointed onto this rectangle raster. Rockworks is a program intended to create 3D or 2D models of the subsurface, based on the information of single boreholes. The important information which have to be given for each borehole in order to be able to create an accurate 2D model with this program are the borehole relative positions and their elevation. The positions do not necessarily need to correspond to the coordinates of the boreholes. The use of a rectangle raster and the relative positions of each borehole is useful for the model. The user can create the illustrational motives which will be used for each type of geology, one can also create geological formations and divide them to separate stratigraphic units. Additional information such as water level, mechanical properties or seismic properties can be also added to simulate the several properties of the subsurface. The dipping angle and direction of each borehole is also a very important information affects the upcoming model for the subsurface. In the case of the restoration project of 2010, 10 out of the total 24 exploration boreholes had a 45° inclination. The cross-section (red line) created by the combination of the 24 exploration boreholes is normal to the general dipping and along the area where the hard coal, of the coal seam Girondelle, was exported (Fig. 5).

The yellow raster on the top of Fig. 6 is the same as the raster on Fig. 5, where it dictates the absolute position of the boreholes on the google map. The 2D model does not represent the actual situation of the subsurface but it creates an approximation of the situation. According to the model, loose material is to be met below the depth of 16 m below surface. In the current area, the loose material was eventually fixed by cement injections during the restoration of 2010.

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Fig. 6 Above: Raster of the exploration boreholes of the Project 2010. The red line represents the position of the cross-section done by the program Rockworks 2006. Below: 2D model of the subsurface. There are some limitations on the representation of the geological formations (no pinch out, no layer continuity) because the program can only provide illustrative representation of the subsurface.

The geophysical measurements, for the evaluation of the state of an old mine in the area of GZB, took place on the lateral edges of the restored area of 2010 as shown in Fig. 4, where only the restoration project of 1980 took place. The prospection profiles will be normal to the strike of the coal seam. Based on the 2D model made with Rockworks, it can be assumed that loose material, for the area where only the restoration project of 1980 took place, should be found below, at least, 16 m depth. This depth is a key information about the geophysical configuration that should be used so that it can be reached by the measurements.

2. Surveys and Theoretical Background

To increase the resolution and diminish the ambiguity of the results of the geophysical measurements, it is usual to apply more than one prospection methods at a certain area. Each method aims on the characterization of different properties of the subsurface, which, combined, will afterwards result to the final model for the subsurface. The geophysical methods, meant to be used for the evaluation of the state of the subsurface for the current study are the: a) Ground penetrating radar, b) Geolectrical tomography and c) Seismic survey.

The ground penetrating radar (GPR) can be used to detect objects, changes in the material properties, voids and cracks. It is the fastest and easiest method to create pseudo-sections of the subsurface for 2D or 3D imaging. It provides high resolution results for shallow depths and in the case of this study, it will be used to detect the injection points of the restoration project of 1980 on the western working position, because the exact positions were poorly documented into the project’s report. The geoelectrical prospection is used in the next step of the exploration workflow, in order to reach bigger depths. This method is one of the least affected by electromagnetic noise, and thus suitable for measurements in urban areas. Target of the measurements is the detection of cavities and loose material in deeper layers, where the mine excavation was taking place. The seismic survey is used as a final, higher precision step for the evaluation of the subsurface state, since it can reach deep structures with high resolution. Seismic is also mandatory for the survey because it is affected by different ground properties than the other two methods giving additional information for the subsurface representation. All three methods can be used complimentary one to the other to gain high resolution and reliable results. Nevertheless, the study results also depend on a great part on the field work accuracy, the processing of the data and the final interpretation.

2.1 Ground Penetrating Radar Survey

Ground penetrating radar (GPR) surveys are proceeding fast, compared to other geophysical methods. This feature enables measurements to be made quickly and repeatedly, yielding high temporal monitoring resolution. Furthermore, GPR presents a completely non-invasive method. The antennas do not have to touch the ground and thus it does not disturb the natural soil conditions. Ground penetrating radar systems are compact and easy to use compared to other geophysical equipment, allowing for scanning over a wide area for the collection of 2D or 3D data. Further, the distribution of soil properties can be obtained with high spatial resolution (Tahakashi et al., 2012).

Ground penetrating radar, GPR, is a high resolution geophysical method, similar in principle to the seismic reflection technique, except that it is based on the propagation and reflection of electromagnetic waves rather than elastic waves. The GPR method operates by transmitting a very short electromagnetic pulse into the ground using an antenna. The frequency which can be typically used varies between the range of 10- 1000 MHz. Abrupt changes in dielectric properties cause parts of the electromagnetic energy to be reflected to the ground surface, where it is recorded and amplified by the receiving antenna. Changes into the electromagnetic properties can often become related with changes in the geological structures, facies, and/or internal distribution of properties such as porosity and fluids (Leucci, 2008). The knowledge of the constraints on attenuation of electromagnetic waves is important to identify electrical properties of different materials in the ground (in particular, relative dielectric permittivity (RDP) and electrical conductivity) and to determine the maximum penetration depth of GPR (Leucci, 2008).

The penetration depth of the GPR method is controlled by the dominant (central) frequency of the transmitting antenna, the electrical conductivity and the attenuation of the subsurface deposits. The signal attenuation is associated with electrically conductive materials (e.g., clays or ions dissolved in groundwater). Therefore, GPR is well suited for resistive environments. The vertical resolution depends primarily of the wavelength, λ, of the propagating electromagnetic wave, which is determined by the GPR frequency, f, and velocity, v, of the ground material as λ=v/f (BurVal Working Group, 2006). Therefore, high frequency signal, translates to narrower wavelength, yielding a higher time or depth of resolution, as well as lateral resolution. On the other hand, attenuation increases with frequency, therefore high frequency signal cannot propagate deep into the ground and the depth of detection becomes shallower. Therefore, lower frequencies result to greater depth penetration.

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Fig. 7 Zero offset GPR configuration. The transmitting and the receiving antenna are placed on the same carrier and they are moved together along a profile while they transmit and receive the electromagnetic waves. The distance between the two antennas is negligible and it is not changing.

The electromagnetic wave propagation is governed by Maxwell’s equations, and most geological media are mainly influenced by the dielectric permittivity, ε, and the electrical conductivity, σ, whereas the influence of the relative magnetic permeability is generally negligible (Takahasi et al., 2012). These parameters affect both wave propagation velocity (1), ν, and radar energy attenuation (3), α.

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where c is the electromagnetic wave velocity in vacuum (0.3 m/ns), ε=εrε0 the dielectric permittivity and ε0 the dielectric permittivity in free space (8.854·1012 F/m), μr is the relative magnetic permeability (for non-magnetic μr=1), ω=2πf the angular frequency, where f is frequency, and the expression σ/ωε is a loss factor. In nonmagnetic (low-loss) materials, such as clean sand and gravel, where σ/ωε ≈ 0, the velocity of electromagnetic waves can be reduced to the expression:

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The attenuation is proportional to the electrical conductivity, which leads to high attenuation in materials with high electrical conductivity.

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where μ =4π·10-7 henry/m. In low-loss materials, where σ/ωε ≈ 0, the attenuation coefficient is reduced to:

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Once α and v are known, one can calculate the average values of σ and ε by inverting

(1) and (3) as follows:

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where, at high frequencies and/or very low conductivity, we have:

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the common situation with both a current source and a current sink the current flow lines and the equipotential surfaces become more complex (Fig. 8b). In reality the current flow lines and the equipotential lines will form an even more complex pattern as the current flow lines will bend at boundaries, where the resistivities change (Ernston & Kirsch, 2006). The penetration depth depends on the chosen electrode array, the maximum electrode spacing and the data density. Furthermore, the resistivity structure in the ground and the contrast of the resistivity structures influence on the penetration depth. A conductive layer close to terrain tends to decrease the penetration depth since the current primarily would flow in the conductive layer. If a resistive layer overlies a conductive layer the penetration depth may increase as the current tends to flow in the deeper conductive layer (Ernston & Kirsch, 2006).

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Fig. 8 Simplified current flow lines and equipotential surfaces arising from (a) a single current source and from (b) a set of current electrodes (a current source and sink) (BurVal Working Group, 2006).

Some of the most common electrode arrays are dipole-dipole (Fig. 9a), Wenner (Fig. 9b) Schlumberger (Fig. 9c), pole-pole and pole-dipole array. Recently, the gradient array (Fig. 9d) has gained renewed interest, since it is well suited for multichannel systems. The dipole-dipole, pole-dipole and the gradient arrays have been proved through a

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Fig. 9 The most common geoelectric configurations used for a resistivity survey. The letters A and B denote the current electrodes an M and N denote the potential electrodes. (modified after BurVal Working Group, 2006).

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2.2 Geoelectric Survey

Direct current (DC) electrical resistivity surveying is a popular geophysical exploration technique because of its simple physical principle and efficient data acquisition. Traditional resistivity measurements are carried out on the Earth’s surface with a specified array to obtain apparent-resistivity sounding curves, apparent-resistivity profiling data or apparent resistivity pseudo-sections, all of which qualitatively reflect the vertical or horizontal variations in subsurface resistivity. This technique is widely used in groundwater, civil engineering and environmental investigations. It is carried out by recording the electrical potential arising from current input into the ground with the purpose of achieving information on the resistivity structure in the ground (Ernston & Kirsch, 2006). The structures that can be depicted through this method range a millimeter scale to kilometers (Storz et al., 2000). In a homogeneous ground (halfspace) the current flows radially out from the current source and the arising equipotential surfaces run perpendicular to the current flow lines and form half spheres (Fig. 8a).

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Details

Title
Evaluation of an Old Coal Mine in the area of GeothermieZentrum Bochum by Geophysical Prospection
Subtitle
Field Measurments in Search of Empty Spaces or Weathered Material in the Subsurface
College
Ruhr-University of Bochum  (Geology and Minerallogy)
Course
Geophysical Prospection
Grade
2.0
Author
Year
2016
Pages
60
Catalog Number
V365484
ISBN (eBook)
9783668483217
ISBN (Book)
9783668483224
File size
5372 KB
Language
English
Tags
geophysics, geophysical prospection, georadar, geoelectric, BERT, DC2DInvRes, seismics, FMTOMO, coal mine inspection
Quote paper
Anastasia Kokkinou (Author), 2016, Evaluation of an Old Coal Mine in the area of GeothermieZentrum Bochum by Geophysical Prospection, Munich, GRIN Verlag, https://www.grin.com/document/365484

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